A debate raged for many years over the properties of light. The question was "is light a wave or a particle?"

The photoelectric effect demonstrated that light was a particle. This experiment used light to liberate electrons from a metal plate, thus allowing electricity to flow between two metal plates in a vacuum tube. This proved that photons of light existed.

However, another experiment showed that if you beamed light through a grid the resulting interference patterns were very similar to the interference patterns of water ripples passing through a grid. This showed that light behaved as a wave.

So which was correct? The proposed solution was that both were correct, known as wave-particle duality. This is illustrated in the two slits experiment.

Imagine that you have a box with two narrow slits cut in the front of it. On the wall of the box opposite the slits you have some form of light detector (e.g. photographic film)

When light is shone though the slits the waves of the light interfere. This is exactly the same as waves in water or sounds waves interfering. All that is happening is that if the peak of one wave meets the trough of another wave in the same place then they cancel each other out (called destructive interference). If two peaks or two troughs meet then the result is a peak or trough of a greater magnitude (called constructive interference).

This interference is obviously only possible with waves (that have peaks and troughs) and not particles (which don't).

Thus when the light is shone through the two slits the expected interference pattern is observed.

Now imagine that you turn the intensity of the light down very low, so low in fact that only one photon of light is emitted a minute. In this case the photon can only go through one slit or the other. If there is not light going through both slits, then there can be no interference and thus no pattern. However, incredibly an interference pattern is observed (if you leave the experiment running for a while!).

This implies that each photon has gone through both slits simultaneously! Very counter-intuitive and scary if you ask me, but nonetheless it happens…

There is more: when detectors are added to the experiment to try and determine which slit the particle goes through it is observed only going through one.

The implication here is that light is whatever you look for it to be: if you look for a particle, it is one, and if you look for wave interference patterns you find them as well.

A further important conclusion is that how you try to measure or investigate affects the outcome



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